One existing mobile communications network is a UMTS Terrestrial Radio Access Network (UTRAN), and a development of this is the Evolved UTRAN (E-UTRAN) network. In the downlink, an E-UTRAN network will support several multiple input multiple output (MIMO) schemes including multi-user (MU) MIMO techniques. The MU-MIMO technologies have also been widely adopted in other future wireless communication standards e.g., IEEE P802.16-REVd/D5-2004, “Part 16: Air Interface for Fixed Broadband Wireless Access Systems”. Furthermore MIMO schemes can be used in both uplink and downlink. The UTRAN system will also employ a hybrid automatic repeat request (HARQ) scheme, either asynchronous or synchronous (S-HARQ) either in the uplink or downlink or in both directions. The HARQ operation should work in conjunction with the MU-MIMO in either direction while meeting the requirements of HARQ process and at the same time fulfilling any MU-MIMO relation conditions.
As is well known, the term MIMO implies that both the base station and the User Equipment (UE) have multiple antennas. MIMO then provides different spatial processing which has the potential to contribute significantly to improve spectral efficiency, diversity, coverage, interference mitigation, etc. MIMO modes or techniques can be categorized in a number of different ways. One type of classification is based on whether users can be served simultaneously or not. As this type of classification is more relevant to this invention, therefore the following two types of MIMO modes are described in more detail:                Single user MIMO        Multi-user MIMO        
In a single user (SU) MIMO scheme all MIMO streams are assigned to a single user at a time. This means a user can achieve very high peak data rate. However, this approach is feasible when a single user has sufficient data traffic in its buffer at the base station and all the MIMO streams exhibit sufficiently good channel quality. Typically single user MIMO provides higher gain in less dispersive channel environment.
In a multi-user (MU) MIMO scheme several UEs are assigned the same resource block(s) on different MIMO streams at a time. This scheme is more useful when there is large number of simultaneous active users in the system and they don't require very high peak data rate. The obvious solution is to share the downlink resources among these active users. MU-MIMO provides higher performance gain compared to the SU-MIMO at higher system load. MU-MIMO is also more flexible especially with respect to delay sensitive services since several users can be scheduled at the same time. However, simultaneous transmission requires that orthogonality conditions between the users to be scheduled are fulfilled as described further below.
The base station has to simultaneously schedule UEs in such a way that multi-user interference is minimized. Otherwise the potential gain of MU-MIMO could be lost. Thus, the UEs with orthogonal pre-coding vectors (W) are scheduled simultaneously to reduce multi-user interference. There is then a performance gain for scheduled users with orthogonal beams over the users without orthogonal beams.
The orthogonality conditions can be explained further in a case where:                W1 is orthogonal pre-coding vector for UE1        W2 is orthogonal pre-coding vector for UE2        W3 is orthogonal pre-coding vector for UE3IFW1*W2′=0, W1*W3≠0Then        UE1 and UE2 can be scheduled simultaneously        UE3 cannot be scheduled with UE1 and UE2 simultaneously        
In other words some UE could not send until the orthogonality condition is satisfied, e.g. pre-coding vector orthogonal to other schedule UEs.
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which are combined with Forward Error Correction (FEC), called Hybrid ARQ. A hybrid automatic repeat request (HARQ) protocol performs both backward and forward error correction of the transmitted packets. The backward error correction is characterized by the retransmission of the packets or HARQ protocol data unit (HARQ PDU) by the transmitter in response to the reception of negative acknowledgement (NACK) from the receiver. The forward error correction is performed at the receiver by making use of the redundancy and the retransmitted packets or PDU.
The hybrid ARQ (HARQ) can be either synchronous or asynchronous. For both asynchronous and synchronous HARQ, it is assumed that the ACK/NAK signalling is synchronous. The difference of both schemes lies in the timing relationship of retransmissions. For a synchronous concept, the retransmission is performed at a pre-specified time-instance while in the asynchronous case the time of the retransmission is determined by the scheduler and is in general not known by the receiver. The timing relationship of (re)transmission and feedback signalling, e.g. ACK/NACK for synchronous HARQ is explained as below in detail.
FIG. 1 illustrates the synchronous HARQ (S-HARQ) operation, showing transmissions from the base station (BS) and user equipment (UE1) in a mobile communication network.
Data packets (Block 1.1, Block 2.1, Block 3.1) are sent in order, and the user equipment sends either an acknowledgement (ACK) of successful reception of the relevant packet, or an indication (NACK) that the packet has not successfully been received. In the event of unsuccessful reception, the base station retransmits the packet (Block 1.2). This retransmission occurs at a pre-scheduled instant, e.g. at a time T1 after the original transmission.
Hence there is a fixed time between transmissions and retransmissions.
On the other hand in case of asynchronous HARQ, there are no fixed timing relations between the transmissions and retransmissions.
The synchronous HARQ operation can be employed both in the downlink and uplink. In the case of the downlink synchronous HARQ protocol, as shown in FIG. 2, the packets (Block 1.1) are transmitted by the base station (BS) and the acknowledgement (ACK) and negative acknowledgement (NACK) are sent by the UE on the uplink control channel.
As mentioned above, there is a fixed retransmission duration, indicated by T1 in FIG. 2. The time taken for UE1 to receive an HARQ protocol data unit (PDU) from the base station BS is indicated by T2 in FIG. 2, and T3 indicates the duration of uplink scheduling (for ACK/NACK) for UE1.
Thus, in order for the S-HARQ to work successfully, the base station has to receive the ACK/NACK message before the fixed retransmission duration expires, i.e.:T1>T2+T3.
Any other UE feedback needed for link adaptation, e.g. a channel quality indicator (CQI), code book, etc should also be available before the expiry of T1.
In the case of the uplink synchronous HARQ protocol, as shown in FIG. 3, the packets (Block 1.1) are transmitted by the user equipment UE1 and the acknowledgement (ACK) and negative acknowledgement (NACK) messages are sent by the base station BS on the downlink control channel.
As above, there is a fixed retransmission duration, indicated by T1 in FIG. 3. The time taken for BS to receive an HARQ protocol data unit (PDU) from UE1 is indicated by T2 in FIG. 3, and T3 indicates the duration of downlink scheduling (for ACK/NACK) to UE1.
Thus, in order for the S-HARQ to work successfully, the UE has to receive the ACK/NACK message before the fixed retransmission duration expires, i.e.:T1>T2+T3.
In WCDMA synchronous HARQ is used in the uplink, i.e. for enhanced uplink (EUL) transmission. But asynchronous HARQ is used in the downlink, i.e. for high speed downlink packet access (HSDPA) transmission.
In E-UTRAN the current working assumption is to follow the same approach as in WCDMA. This means S-HARQ will be used in the uplink and asynchronous HARQ will be used for downlink packet transmission.
In addition MU-MIMO will be used at least in the downlink of E-UTRAN even in the early phase of standardization. Thus the combination of DL MU-MIMO and UL S-HARQ needs to work properly while retaining their benefits.
In E-UTRAN the discontinuous reception (DRX) and discontinuous transmission (DTX) shall be used even when UE is in connected mode. The purpose of DRX/DTX is to save the UE battery consumption.
In order to complete the HARQ process as quickly as possible, the UE shall enter into continuous reception mode (i.e. non DRX) after the reception of the first HARQ PDU or packet. This means UE should be able to continuously monitor any control signalling and/or packet just after it has received the first packet.
As explained above the S-HARQ operation requires fixed timing between the first transmission and retransmission or between the retransmissions, and for the asynchronous HARQ the time of the retransmission is determined by the scheduler. But for both, asynchronous and synchronous HARQ, it is assumed that the ACK/NACK signalling is synchronous. The major problem arises when MU-MIMO is used with HARQ. MU-MIMO transmission requires the fulfillment of the orthogonality conditions between the scheduled users. Since orthogonality conditions may not be fulfilled for all the active users, i.e. users whose HARQ protocol is in operation. This will lead to a situation where the ACK/NACK or any other relevant control information is not received by the transmitter from the receiver within the due course. This will eventually delay the retransmission beyond the stipulated time, thus collapsing the HARQ operation.
To summarize, in case of UL HARQ, e.g. S-HARQ and DL MU-MIMO, which is more relevant scenario with respective to the standard, the following problem will occur: The network may not be able to send ACK/NACK to all users at a fixed (standardized) time as required by S-HARQ.
In case of DL HARQ, e.g. S-HARQ and UL MU-MIMO the following problems will occur:—
The network may not be able to schedule all users to send ACK/NACK to the base station at a fixed (standardized) time.
All users may not be able to feed back downlink channel state information such as channel quality indicator (CQI), code book etc to the base station at a fixed (standardized) time.